Friday, February 14, 2014

Co-evolution: Moths vs. Bats

Quintessential Creatures of the Night

Mammals are comprised of many different shapes and sizes and are one of the most successful vertebrate classes ever to have shared this planet. Two orders comprise the majority of all mammalian species. The largest order is Rodentia which consists of rats, mice, squirrels and even the massive capybara. Rodents make up 40% of all living mammals. The second largest group of mammals, and my personal favorite (thanks to Dr. Carter at Ball State University), is Chiroptera, or the bats!

Bats come in all sizes. The Bumblebee Bat (Craseonycteris thonglongyai) weighs a hefty 2 grams while the Golden-crowned Flying Fox (Acerodon jubatus) tips the scales at 1.2 kg (edgeofexistence.org; iucnredlist.org)! The former bat is a member of a group of bats commonly referred to as the Microchiroptera, the focus of this specific blog. Microchiropterans are special in that they are experts at echolocation, a skill only a few groups of animals have mastered (toothed whales and some moths for example).

A little bit on echolocation

Echolocation works a lot like it sounds: a sound reverberates off an object in the distance and bounces back to the receiver. How does that help or benefit the bat, though? The bat is able to glean a lot of information from the returning "echo." They can get distance, size, and speed from returning sounds (see here for more on echolocation). These sounds can be extremely high-pitched. Some bats emit frequencies as high as 120 kHz, whereas humans can only hear a maximum of 20 kHz. Most bats that echolocate are using it to capture insects. Just like any prey species, these insects don't want to be eaten. They would rather stay alive to pass on their genes, so it is not surprising to hear that some moths can jam the bats radar...with their genitals! Unfortunately I could not gain access to the specific paper referenced in that article, but we can still talk about bat-moth aerial dog-fighting!

A co-evolutionary arms race

As previously mentioned, echolocation is the bat's primary weapon. We can only imagine how many hundreds of thousands, possible even millions of years that bats had exerted their aerial tyranny over those wimpy moths. But, there was a glimmer of hope. It is not clear what evolved first, the moth mating call or the bat-jamming effects it has, but it is more parsimonious to say that moth mating calls eventually provided protection from bats in a form of stealth or radar-jamming fashion. Moths jamming bat sonar is not really news. Researchers have been studying this topic since the 70's, but new research has revealed an increasing number of moths that use this technique Nakano et. al (2009) characterized moth mating calls in 9 species that were only emitted when in close proximity to females. They proposed that these calls were inaudible to in-elegible receivers. This implies that the moths use separate clicks to remain undetected or hard to detect by the bats.

Physiology of Sound

Bats echolocate via laryngeal echolocation (Jones & Teeling 2006). They also coordinate muscles in the ear and larynx to possibly stabilize the signal (Jen & Suga 1976). Basically, bats are badasses at finding their way around in the dark (Eyeballs?!?!? We don't need no stinking eyeballs!!!). Moths, on the other hand, use a variety of body parts to make sounds. Some use their sexy parts to make sounds while others use a part of their body called the tymbals which are located on the ventro-lateral surface of the thorax (Gwynne 1986; Corcoran et. al 2009). The video below shows the tymbals at work. If you look closely, you can see ridges on a thin membrane and on the inside their seems to be some sort of machinery rubbing past those ridges, working much like a washboard.

Physiological Ecology of the Clicks

So how exactly do these sounds cause a bat to miss when approaching a clicking moth? There are 3 mechanisms that have been proposed. The first claims that the clicks function as "phantom echoes" which the bat perceives as a sound reverberating from its own clicks. Second, the ranging interference hypothesis which states that the clicks degrade the bats signal which makes it difficult for the bat to determine distance. Finally, the masking hypothesis says that the moths basically become invisible, a type of moth stealth-mode if you will. In an experiment with juvenile Big Brown Bats (Eptesicus fuscus) naive to moth clicks, it was clear that the bats were missing targets at a distance, supporting the ranging interference hypothesis (Corocoran et. al 2011). In addition, these same bats reversed their typical call when detecting a moth. If a moth did not click, the bats reduced call duration, but increased the number of calls after detection at close range (often called a feeding buzz). If the moth did click, the bats did not reduce call duration, much like they were confused as to the distance of the moth (exactly what the ranging interference hypothesis predicts)(Corcoran et. al 2011). This tells us that the clicks produced by the moths give the bats a false reading, however, the bats can habituate fairly "clickly" and are then immune to the clicks created by the moths.

Corcoran et. al 2011

If bats can habituate, then why has this radar-jamming persisted? Some researchers claim that the acoustic signal was a form of aposematism, telling bats that eating a moth would be a nasty experience (Corcoran et. al 2009). But bats still attack the moths, so if this were the case, bats shouldn't attempt to eat the moths. This is why an acoustic aposematic signal could evolve into a radar-jamming weapon that would allow moths to temporarily evade bats and live to reproduce (Hristov & Conner 2005). Bats appear to continue pursuit after the moth makes it clicks, supporting the hypothesis that moths jam bat radar. However, sometimes these bats have been observed spitting out moths that click because they were unpalatable (Hristov & Conner 2005). Being that some clicking moths are eaten after being captured by a habituated bat and others are not, this suggests that some moths have an honest signal of unpalatibility while others have used Batesian mimicry in an attempt to avoid a predation attempt. Bats have figured this out and basically just attack everything now and spit it out if they don't like how it tastes. Your move, moths.